Add nHighTransactionFeeWarning as per #3969.
Organize RPCCommands table

Use sensible categories (overall control, P2P, blockchain/UTXO and
mining, wallet, wallet-enabled mining) and sort within each.

Also remove unnecessary #ifdef ENABLE_WALLET from `rpcnet.cpp`.

Functionality-neutral change.
Merge pull request #3964

ab88ed9 Organize RPCCommands table (Wladimir J. van der Laan)
Merge pull request #3969

ffeb473 Add nHighTransactionFeeWarning as per #3969. (Bardi Harborow)
Fix typo in `createmultisig` help

iCreateMultisig is not a thing.
Merge pull request #3987 from laanwj/2014_03_icreatemultisig

Fix typo in `createmultisig` help
Wrap create_directory calls in try...catch blocks.

Ignores any exceptions thrown if directory exists, otherwise re-throws exception.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>